You wouldn’t need plausible deniability because the prosecution would have to prove that your intent was to make the bill unfit for circulation. Intent is already notoriously difficult to prove in a court of law, and it would be very difficult to prove someone wanted to take a bill out of circulation by writing a message on it that they hoped would be seen by people.

Even if the result is that the bill gets taken out of circulation, the court would have to prove that you knew that would happen and wanted it to.